1. Field of the Invention
The present invention relates to the field of power supply circuits, DC-DC converters, low dropout regulators, synchronous rectifiers, line drivers, voltage regulators, pass elements and the like.
2. Background
A typical step-up DC-DC converter employing a flyback topology is shown in FIG. 1. In this circuit topology, an inductor L1 alternately stores and releases energy which is directed to the load in a controlled manner. During the charging cycle, the switch S1 is turned on. The voltage across inductor L1 induces a current which rises linearly until the voltage is removed by opening switch S1. When switch S1 is opened, the stored energy in inductor L1 attempts to maintain the current therethrough, causing the voltage across inductor L1 to change polarity. The voltage at the anode of a catch diode D1 (hereinafter referred to as the voltage at LX, or VLX) rises until diode D1 is forward biased, delivering power to the output. The open and closed positions of switch S1 are determined by a switch control circuit 12 which is designed to keep the output in regulation.
There are two main drawbacks with using a simple catch diode in battery powered systems.
First, because catch diode D1 presents a low impedance path to the output even when control circuit 12 holds the switch S1 off, a conventional DC-DC converter may drain the battery coupled to its input and consume power even during a shut-down mode. With the control circuit holding switch S1 off, the output voltage is at VIN-VF (VF=the forward-biased voltage drop of diode D1), and the output load, if not switched off, drains the battery. Typical solutions have been to either place a switch in series with the load or to have a power-down mode for the load.
Second, the battery voltage can vary over a wide range from a voltage level when the battery is fresh to another voltage level when the battery reaches its end-of-life or end-of-charge condition. Thus, it is possible for the output voltage to be within this range, making the design of the DC-DC converter difficult. For example, a four-cell (Alkaline) input to 5V output is a popular application. The operating input voltage can range from 6.2V when the battery is fresh to 3.6V at the end of the battery life.
In these applications, the use of a typical boost converter with a catch diode presents a problem since when an input voltage to the catch diode is greater than 5V+VF, the output voltage will be VIN-VF, and thus, the output voltage will be out of regulation. A paper titled "Get +5V/100 mA From Four Cells," Electronic Design, Jan. 9, 1992, p. 132 describes an approach to boost the input voltage to above the maximum expected input voltage and to use a linear regulator to regulate the voltage down to a desired voltage. Such a conventional approach is not only complicated but also low in efficiency.
The present invention includes a synchronous rectifier which overcomes these drawbacks of a catch diode, especially in battery powered systems.
The present invention provides a step-up and step-down DC-DC converter having an on-chip synchronous rectifier. This synchronous rectifier replaces the Schottky diode of a conventional step-up DC-DC converter, thus reducing the number of external components and providing a lower forward voltage drop across the synchronous rectifier than that across a Schottky diode. In the present invention, the on-chip synchronous rectifier appears as a high impedance element in series with an inductor and a load when the device is placed in a shut-down mode. This high impedance prevents the current drain associated with conventional step-up converters in the shut-down mode.
The step-up or step-down DC-DC converter of the present invention can maintain regulation of the output voltage even when the input voltage is significantly above or below the target output voltage. For example, the present invention can generate and regulate a 5V output from 4 Alkaline batteries when the input voltage to the DC-DC converter varies from 6.2V, or 1.55V/cell with fresh batteries, to 3.6V or 0.9V/cell at the end of the battery life.